1. Field of the Invention
This invention relates to a coated paper for printing that is light in weight yet offers particularly excellent appearance in the unprinted state along with equally excellent printability.
2. Description of the Related Art
There has been a strong demand of late for high-quality, coated papers for printing suitable for printed matter containing many photographs, illustrations and colors, given the desire to convey the printed content with strong visual impact. On the other hand, there is also a strong demand to reduce the weight of printed matter as a means of saving energy and the costs of shipping and mailing. These two basic demands are mutually exclusive, however, since high-quality, coated papers for printing use a heavier base paper and more coating material and are more expensive, thus failing to meet the need for light weight and low cost. This has given rise to a need for technology that achieves higher printing quality using a so-called “low-grade” coated paper having low basis weight and coating weight.
Whiteness, opacity, sheet gloss, printed gloss and stiffness are particularly important among the qualities required of a coated paper. Whiteness enhances contrast, while opacity prevents the content on the back face of the printed page from showing through. Gloss affects the quality feel of the printed matter. It is essential that all these properties be satisfied while maintaining an optimal balance. Stiffness mainly affects printing efficiency and is an important factor that determines the ease with which the printed pages can be turned.
Coated papers are largely classified into glossy coated papers and matte-coated papers. Glossy coated papers include art papers and high-grade art papers used in the printing of expensive publications, and other coated papers used for printing catalogues, magazines, brochures, and so on. These papers, with their glossy finish, offer excellent gloss in both the unprinted and printed areas of the finished product. Matte-coated papers include those of dull finish and matte finish and offer lower gloss in both unprinted and printed areas. Matte finish makes the printed product look flat and heavy due to its low gloss. Dull finish has a property intermediate between glossy and matte finish, as it offers low sheet gloss but high printed gloss.
As mentioned above, reducing the weight of a coated paper for printing requires a reduction in the basis weight of the base paper and a reduction in the weight of the coating layer. However, simply reducing the basis weight without changing the composition of the paper material will reduce the paper thickness in proportion to the decrease in the weight of the paper, resulting in insufficient opacity and stiffness. Providing a thick coating layer on a base paper of low basis weight will retain the same level of printability achieved with-a base paper of normal basis weight, but this approach is not practical because the opacity and stiffness will drop. Additionally, such a measure will reduce the tensile stiffness, possibly leading to problems during offset web printing such as torn paper on the turning cylinder, at the paster, or when the press is started.
Providing a coating layer with low coating weight on a base paper of high basis weight achieves sufficient opacity and stiffness. However, reducing the basis weight of a coated paper through this method will require a substantial reduction in the coating weight, in fact to a degree below what is necessary. That results in insufficient printing quality, so therefore the approach is impractical. In other words, a certain balance must be maintained between basis weight and coating weight, requiring a combination of low-basis-weight base paper with a coating layer with low coating weight, or a high-basis-weight base paper with a coating layer with high coating weight. Generally, papers with lower basis weight offer lower whiteness, opacity, sheet gloss, printed gloss and stiffness compared with those of higher basis weight.
Generally speaking, opacity drops dramatically when the basis weight becomes 80 g/m2 or less. To minimize the drop in stiffness and opacity while reducing the basis weight to a certain extent, a lower-density, higher-bulk paper should be produced. Such paper can be made effectively using mechanical pulp from certain types of trees such as gumwood, maple and birch. However, limiting the types of usable trees is not practical from the viewpoints of energy and economy. At any rate the use of mechanical pulp, regardless of the source trees, helps increase stiffness and opacity. Methods known to reduce basis weight and improve opacity include the addition of hollow synthetic resin capsules (Japanese Patent Publication No. 52-118116), and the addition of a synthetic organic foaming filler (such as EXPANSEL by Nippon Filant) and causing the filler to foam in the drying stage. However, hollow synthetic resin capsules and foaming fillers require a complex process of adjusting the mixing and foaming conditions, and they are also expensive. Therefore, these are not presently seen as practical methods. Instead of using a filler, Japanese Patent Application Laid-open No. 8-13380 proposes a method to add micro-fibril cellulose. However, this method is also impractical, because it requires that micro-fibril cellulose be prepared separately, thus complicating the operation. It is known that the addition of fillers, particularly titanium dioxide, to the base paper increases opacity. However, it increases density while reducing stiffness and paper strength. These factors prevent us from simply adding fillers.
As explained above, the simple application of the prior art will not provide a lightweight coated paper for printing that possesses the desired characteristics.